Characterization of Cu 2+ sites in zeolites NaX and KX by 27Al electron spin echo envelope modulation

Abstract

The 27Al FT-ESEEM spectra of Cu 2+-doped NaX, KX and KA zeolites were measured as functions of dehydration temperature and correlated with the observed ESR spectra and the Cu 2+ locations within the zeolite structure. Partial dehydration of CuNaX at 25–300°C produced species B which was assigned of site I in the hexagonal prism, characterized by an 27Al FT-ESEEM spectrum of a single line at the 27Al Larmor frequency (3.5 MHz), indicating that the interaction between the unpaired electron and the 27Al nuclei is governed by weak dipolar interactions. Species A, on the other hand, observed in fully dehydrated CuNaX and in CuKX dehydrated above 100°C is characterized by an FT-ESEEM spectrum showing a doublet at 2.6 and 5.3 MHz which is a result of a rather large isotropic hyperfine coupling of ≈ 2.7 MHz. The large coupling is indicative of stronger binding of the Cu 2+ to framework oxygens in sites III of III′ in the supercage.